fix(go): bound retarget wait and warn on Sepolia DKG fragility

[piotr-roslaniec]

Stack Context

Follow-up to #4002 (stack/testnet4-04-go-testnet4-runtime).
Base: stack/testnet4-04-go-testnet4-runtime

Addresses three runtime/operability issues surfaced during review of #4002.

What's in this PR

1) Bound the synchronous retarget wait — pkg/chain/ethereum/bitcoin_difficulty.go

waitDeployBackendTransactionMined previously called bind.WaitMined(context.Background(), …) with no timeout, then BlockCounter.WaitForBlockHeight (which has no context parameter). A stalled RPC or chain that stopped producing blocks would hang the maintainer indefinitely on every Retarget / RetargetWithRefund call — including on mainnet.

Both waits are now bounded under a shared 10-minute deadline. A small waitForBlockHeightCtx shim adapts the context-less BlockCounter interface, with a regression test verifying it returns context.DeadlineExceeded when the counter blocks forever.

2) Warn loudly about Sepolia DKG fragility — pkg/tbtc/tbtc.go, pkg/chain/ethereum/tbtc.go

The Sepolia/Developer defaultGroupParameters returns {GroupSize:3, GroupQuorum:3, HonestThreshold:2}. Quorum equals size — a single offline operator prevents DKG progress.

• Elevated the existing Infof to Warnf and spelled out the operational consequence.
• Added a Warnf in pkg/chain/ethereum/tbtc.go where the EcdsaDkgValidator ErrAddressNotConfigured branch previously fell through silently. Operators now see at startup whether group sizing is coming from on-chain values or compile-time defaults.

No sizing values are changed.

3) Document the testnet3 → testnet4 wiring as breaking

No code change in this PR; the startup banner at config/config.go:101 already prints the resolved Bitcoin network. Operator-facing note is captured here and should be folded into release notes when #4002 lands.

After #4002 lands, network.Type=Testnet (Sepolia) resolves to bitcoin.Testnet4 rather than bitcoin.Testnet (testnet3). Existing Sepolia operators will switch both the embedded Electrum URL set and the Bitcoin network on upgrade. Pin to a pre-#4002 build to remain on testnet3.

Test Plan

• go build ./...
• go vet ./pkg/chain/ethereum/... ./pkg/tbtc/...
• go test ./pkg/chain/ethereum/...
• go test ./pkg/tbtc/
• New regression: TestWaitForBlockHeightCtx_DeadlineExceeded, TestWaitForBlockHeightCtx_ReturnsImmediatelyOnSuccess

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